Shunt Capacitor Switching For Power Factor Improvement Clayton H Reid.
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Transcript of Shunt Capacitor Switching For Power Factor Improvement Clayton H Reid.
Shunt Capacitor Switching For
Power Factor Improvement
Clayton H Reid
Power Factor
Kw is productive power
Kvar is non productive
Industrial Plant Electrical Load
• Induction Motors
• Induction Furnaces
• Fluorescent Lighting
Advantages of Installing Capacitors
• Improved Power Factor
• Released System Capacity
• Improved Motor and Lighting Performance
• Reduced Current and Losses
• Decreased Transformer Losses
Shunt-capacitor Banks
• Automatic switching of capacitor banks • Voltage Control-a voltage sensitive relay is
used which responds to changes in line voltage• Current Control-a current sensitive relay is used
which responds to changes in line current• Kilovar Control-a kilovar relay is used which
responds to changes in reactive loads
Capacitors Switched with Motor
• Another means of obtaining automatic switching is to connect the capacitor to the motor and switch the motor and capacitor as a single unit
Capacitors Switched with Motor
• The importance of selecting the correct size of capacitor to be switched with a given motor load
• Location of capacitor connected points
• Capacitor switching for special motors and for special motor-starting applications
Capacitors Switched with Motor
• Transient inrush current and frequency for the following cases:
• When a single capacitor is energized on a system
• When a capacitor is energized in parallel with capacitor banks already connected
• Effect of transient currents on contactors• Use of air-core reactors to limit transient
current in parallel switching of capacitors
Overvoltage Due To Excessive Capacitance
• Capacitor connected to the motor and starter de-energized, motor acts as an induction generator with shunt capacitor excitation
Maximum Voltage Generated
• Size of capacitor• Speed of motor• No load characteristics
Overvoltage Due to Excessive Capacitance
Magnetizing Current
Torque Transients
Location of Capacitors
Energizing a Single Capacitor Bank
Capacitor Inrush Current
Transient Frequency
- transient frequency - power frequency
Recommended Capacitor Rating
Inrush Currents
Energizing Additional Banks
Capacitor Inrush Current
CaLa2 Ep
Ip =
Ip= peak in rush current in amps
Ep= r.m.s phase voltages in volts
Ca= total circuit capacitance in farads
La= total circuit inductance in henries
Between C1 and C2
Transient Frequency
Contactor Switching Capability
Transient Overvoltage
Methods Of Limiting Inrush Currents
Method Of Limiting Inrush Current
Capacitor Tests
Air-Core Reactor Design
Air-Core Reactor Design
Air-Core Reactor Design
Capacitor Switching Tests
Single 10 kvar capacitor
Parallel switching of 10 kvar capacitors
Parallel switching of 10 kvar capacitor with reactors
Inrush Current
725 A
Inrush Current
1153 A
Inrush Current
595 A
Transient frequency
1057 Hz
Transient frequency
3340 Hz
Transient frequency
1750 Hz
Summary
• Capacitor selection can be made from manufactures literature. Will provide correction to approx. 95% lagging, voltage will be limited to 110% when motor disconnected.
• Capacitors should be connected ahead of overload relays. If connected after the relays Overload section should be selected based on reduced current through the relays.
• Do not connect capacitors to the winding of a motor driving a high inertia load.as torque transients up to 20 times can occur resulting in mechanical damage to motor shaft and driven machinery
Summary
• To avoid torque transient problems for motor and driven machinery,capacitors should not be connected directly to the motor in the following :
• a) any open transition reduced voltage starter• b) reversing starters, or starters which are used
for for jogging the motor• c) two speed motors• d) wye-delta motors• Use a separate contactor to switch the capacitor
Summary
• When capacitors are installed in motor control centers additional inductance should be installed in series with the capacitors to limit transient charging current.This will reduce contact erosion in the contactor